The invention relates to a method for the production of a hot-rolled steel strip in bundles or in sheets from a steel melt in a continuous manufacturing process, with an uninterrupted run through a combined casting and rolling plant, and to a combined casting and rolling plant for carrying out the method.
A method of this type comprises the following method steps: a steel strand is formed from liquid steel by continuous casting in a continuous casting mold of a continuous casting plant. In a following first group of roll stands, this cast steel strand is roll-formed into a pre-rolled hot strip, and, in a second group of roll stands, this pre-rolled hot strip is finish-rolled into a hot-rolled steel strip having the desired final dimensions and the desired material properties. Between the first group of roll stands and the second group of roll stands, a setting of the pre-rolled hot strip to the rolling temperature takes place in a temperature setting device, in order to achieve beneficial conditions for the finish-rolling. After finish-rolling, the hot-rolled steel strip runs through a cooling zone and is wound into bundles or divided into sheets.
Methods and arrangements of production apparatus for producing a hot-rolled steel strip starting from the liquid phase, which may come under a few basic types of method, are known in many variants from the prior art.
In a discontinuous process for generating a hot-rolled steel strip, liquid steel is cast on a continuous casting plant into a continuous steel strand, and the latter is divided into slabs having a casting thickness of more than 120 mm or into thin slabs having a casting thickness of between 40 mm and 120 mm. Subsequently, or with a time interruption as a consequence of production, these preproducts are rolled, after temperature compensation or basic reheating to rolling temperature, in rolling plants into steel strip having a specific target thickness. For this purpose, conventionally, a single-stand or multistand roughing train and a multistand finishing train are employed.
WO 98/00248 already discloses a combined casting and rolling plant for the production of a steel strip of deep-drawing quality, in which a steel strand having a casting thickness of less than 100 mm is cast in the continuous casting mold of a continuous casting plant. This cast steel strand is rolled, after descaling has been carried out, in a multistand roughing train at least to a windable strip thickness and, after running through an inductively heated furnace, in which a nonoxidizing protective gas atmosphere is maintained and in which the pre-rolled hot strip is heated to a temperature in the austenitic range, is wound into bundles and stored in an intermediate store. After the bundle has been unwound again, the pre-rolled hot strip is delivered to a finishing train and is rolled into a final product in the ferritic structural range. The rolling speed in the finishing train can be set specifically to the product by decoupling from the casting plant, irrespective of the specific requirements which are to be met by the plant arrangement and which arise from the special steel quality in this prior art. However, due to the winding and unwinding of the pre-rolled steel strip into bundles and the intermediate storage of these, considerably higher investment costs are incurred than in a continuous strip runthrough. A production method decoupled in this way, in which at least the rolling speed in the finishing train can be set independently of the casting speed in the continuous casting plant, is absolutely necessary in a multistrand casting plant. In single-strand casting plants, this decoupling may likewise be carried out and leads to the disadvantages described.
WO 89/11363 already discloses a continuous method of the generic type for the production of a hot-rolled steel strip, in which the casting speed at the exit from the continuous casting mold determines the rolling speed in the following respective forming stages as a function of the degree of forming in the respective forming stage. Only after exit from the last roll stand is the continuously cast and hot-rolled steel strip divided transversely according to the predetermined bundle weight and wound into bundles. Before the hot strip already pre-rolled in a roughing stand enters the finishing train, this hot strip is brought to a uniform strip temperature which lies above the rolling temperature, and is subsequently descaled immediately before entry into the finishing train. Descaling with water jets results in a temperature loss which has to be compensated by preceding heating to a temperature above the rolling temperature.